Multigap superconductivity in the filled-skutterudite compound LaRu4 As12 probed by muon spin rotation

Muon spin rotation (μSR) and inelastic x-ray scattering were used to investigate the superconducting properties of the filled-skutterudite compound LaRu4As12. A two-gap isotropic (s+s)-wave model can explain the temperature dependence of the superfluid density. Zero-field μSR measurements confirm that the time-reversal symmetry does not break upon entering the superconducting state. The measurements of lattice dynamics at 2, 20, and 300 K revealed temperature dependences of the phonon modes that do not strictly follow a hardening of phonon frequencies upon cooling as expected within the quasiharmonic picture. The 20 K data rather mark a turning point for the majority of the phonon frequencies. Indeed, a hardening is observed approaching 20 K from above, while for a few branches a weak softening is visible upon further cooling to 2 K. The observed dispersion relations of phonon modes throughout the Brillouin zone matches with the density functional theory prediction quite closely. Our results point out that cubic LaRu4As12 is a good reference material for studying multiband superconductivity, including those with lower crystallographic symmetries such as iron arsenide-based superconductors.